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Electroless nickel plating is tarnishing, turning black

A discussion started in 1995 and continuing through 2017 . . .


Q. We've run into a bit of a problem of nickel discoloration on some heatsinks.   We build Mil-Spec high reliability computers. In one case the modules has electroless nickel on copper heatsinks. The heatsink looks vaguely like a ladder and is bonded to FR-4 multilayer circuit board (printed wiring board, or PWB). Chips straddle the rungs and are bonded down with epoxy and are soldered into the PWB using RMA rosin flux and a wave solder machine.  

Electroless Plating
Mallory & Hajdu

We installed a semi-aqueous PWB cleaning system and the heatsinks are coming out with black water spots (well, that's a bit over stated, there are water spot shaped areas ranging from clear tan to blackish). We're desperate to find out what's causing the discoloration and more importantly, how to avoid it. (Discoloration of nickel plating does not meet Mil-Spec)   The cleaning process is a batch process with 4 sumps and a drying bay. The first sump is Axarel 46, a DuPont devised, now Petroferm sold hydrocarbon mixture (aliphatic hydrocarbon with dibasic esters: isobutyl glutarate, adipate, succinate and surfactant) - temperature 120 F, ~10-20 minute immersion, immersion spray agitation The second sump is an emulsion of DI water and Axarel - temperature 140 F, 5 minute immersion, immersion spray agitation Third and fourth sumps are DI water, 140 F, 5 minute immersion in each, immersion spray agitation. Final sump is hot air, 190 F, the rack is in the bay for 6 minutes, there is a slit nozzle at the top of the bay to act as hot air knife to blow surface water off. Naturally the problem is mainly showing up on product, not test coupons.

We did reproduce a mild case of the discoloration after taking a chunk of heatsink, bonding it to a PWB, putting epoxy tape on it, fluxing it, running it through the wave solder and then cleaning it. The discoloration appears to occur in the last spots water is standing after coming out of the final rinse before drying. We also tried cleaning a section of heatsink as-received, and after bonding to a PWB. Neither had appreciable discoloration. We sent the as-received and discolored heatsink section out for ESCA analysis of both discolored and non-discolored areas.

The as-received was dirtiest- ~2% Ni at the surface, 80% C, 10% O, smattering of Cu, Pb, Sn, P. Unstained area on stained heatsink - 20% Ni, 56% C, smattering of Cu, Pb, P. Stained area on stained heatsink - 18% Ni, 61 %C, smattering of Cu, Pb, P 40 Angstroms down ESCA shows as-received - 20% Ni, 60%C, smattering of Cu, Pb, P Unstained area - 70% Ni, 20%C, 7%P Stained area - 65% Ni, 25%C, 9%P Visually at 200X the stained area has a transparent brown tint versus the silver surface elsewhere. There are iridescent spots visible over all the surface, perhaps tending to slightly larger spots in the darker areas. (Possibly residual hydrocarbon? - it does have a boiling point of 350 F)  

Anyone have thoughts as to what it is, how better to identify it, or how to prevent forming it? We got a suggestion that electroless typically would have an inhibitor applied to prevent discoloration, is this common? Would the cleaning process remove this?

Steven Axdal
- Bloomington Minnesota


A. If your electroless nickel has 80% carbon on the surface when you receive it, I believe that is the problem. I don't think there should be any post treatment on the electroless nickel. It is not needed, I doubt you asked for it, and that is not MilSpec.

Tom Pullizzi
Falls Township, Pennsylvania

Electroless nickel tarnished with rainbow colors during heat treatment


hill cross


Q. I am an electroplating jobber in Penang, Malaysia. My company provide gold, silver, palladium-nickel, tin, tin-lead and electroless nickel (EN) plating to the electronic and electrical industries.

I had lately doing some EN coating jobber that require heat treatment, the job pieces tarnished with rainbow colors in the heating chamber. What caused this? Could you share some experience with me?

After the EN coating we passivate the surface with dilute chromic acid. Then heated in nitrogen gas purging oven with the following profile:

Ramp up from 140 °F to 750 °F in 30 minutes.
Soak at 750 °F for 75 minutes.
60 minutes to cool down to 140 °F. Remove from oven.

Thank you and best regards,

Keat-Seng Ong
- Malaysia


A. Rinsing before bake must be thorough. Consider using D.I. water.

Richard Painter
- Cleveland, Ohio


A. I agree with the better rinsing. Also, is the 750 °F necessary. What are you trying to achieve, higher hardness?

Robert Sachs
- Canada


Q. I'm putting an electroless nickel finish (approx. .0004" thick) on a machined Aluminum (6061-T6) part that is then glass bead blasted (240 grit) prior to plating. The matte finish is very attractive and consistent but highly vulnerable to tarnishing from handling (hand/finger sweat). Is there any way to alleviate this problem? What is the most effective cleaner to remove the tarnish (preferable a cloth type)?

Thank you,

Duncan Mackay
- Palo Alto, California, USA


A. Most chemical supply houses can offer an anti-tarnish post-dip for electroless nickel. Technic, for example, offers their Tarniban series of post-dips.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Readers: To minimize searching & thrashing, and offer multiple viewpoints, we combined multiple threads into the dialog you are viewing. Please forgive any resultant repetition.


Q. Is it possible to Alodine an electroless nickel plated part (aluminum base metal)? I'm assuming the Alodine finish would lessen finger printing of the nickel and would make these unsightly marks easy to remove, probably with a damp cloth? If this is all feasible then how long should the part be submerged in the chromate solution for the best result?

Thank you.

Duncan Mackay
- Palo Alto, California, USA


A. Anti-tarnish compounds are indeed often applied to electroless nickel, and the composition of those chemicals might be rather similar to the composition of Alodine, which is a trade name of Henkel Surface Technology for chemical processes used in the chromate conversion coating of aluminum. But I doubt that Henkel sanctions this process being used for that purpose, and it is not good QA practice to use non-documented processes. Instead, ask them or one of your other vendors for a anti-tarnish topcoat for electroless nickel.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


Q. We have been using an electroless nickel plated (0.2 mils) aluminum electrical connector for a number of years on a power supply. Our supplier solders wires to it, uses some RTV to hold some of the wires, fills the Tin plated, cold rolled steel box with a heat-activated epoxy and alumina, mechanically attaches the connector to the box, without sealing around it, and heats the box to 80 °C for 4-6 hours. After a few years we are noticing a black "tarnish" that seems to have "creeped" up the walls of the inside of the connector - not all the way. We also see some "white" deposits at the bottom of the connector. We are seeing: Ni, Zn, S, K, Ti, Al in these areas; none of these elements are in the epoxies. Also of note, the customer exposed the power supply to 130C coolant while not operating for two hours. Does anyone have an idea what could be happening? Our customer is concerned about long term corrosion and products of conducting material falling off.

Geoffrey M Smith
Power Supply Design Engineer - Baltimore, Maryland, USA


A. 0.0002 inches of electroless nickel is not thick enough to close all pores in the aluminum device. Corrosion cells start at pores and accelerate by electrochemical cells due to dissimilar metals in contact with the EN plate. A chromate passivation can be useful, but not always effective if the pores are open and the voltage generated by the couple. Don

Don Baudrand
Consultant - Poulsbo, Washington

(Don is co-author of the
book "Plating on Plastics")


A. It's hard for me to see the black tarnish in your photo but we've experienced something similar to what you describe. Our connectors are also electroless nickel plated, however they are made from composite (plastic) base material. They're Class M, 38999 series III circular connectors. Our tarnish ranges from "tea stain" to very dark, opaque black; as if the parts were pulled out of a fire, sometimes accompanied with very small amounts of white powder down in the recesses of the connectors.

We've been able to associate the tarnish with a latex masking operation performed by our Parylene conformal coating vendor. The vendor had been using an ammoniated latex for masking the connector contacts and other mating surfaces. They may also have been speeding the processing time by oven baking or applying the ammoniated latex into connectors fresh out of a drying oven. In any case the tarnish corresponds to the exact locations of the masking. We've asked the vendor to use a non-ammoniated latex in the future, but due to lack of sales on this product, we haven't been able to test this out and ensure this is the cure.

The bulk of the tarnish is removable with rubber eraser abrasion but a "tea stain" appearance remains.

Are your connectors part of a circuit card assembly that is masked for conformal coat?

Does anyone know of a reaction that occurs between nickel and ammonia or some other constituent of latex rubber?

Gregg Temkin
Avionics Manufacturing - Seattle, Washington, US


Q. My company plates a variety of materials with mid-phos electroless nickel. Recently, we have seen some discoloration form as the plated material ages in the form of a black tarnish that retains some of the shine on our copper alloy parts. The parts are rinsed in hot DI water post plate, as all our nickel plated parts are, and none of the other plated parts have shown this malady. Other than surface contamination and reaction in post plating processes, what would cause this to occur (what type of contamination)?

Allen Clague
Plating manager - Waynesboro, Virginia

Electroless Nickel Plating
Wolfgang Riedel

Electroless Copper and Nickel-Phosphorous Plating

simultaneous (2007)

A. Not enough information. The coating may be too thin, the EN solution may have high impurity content, the environment may be too severe (sulfurous?).

The corrosion/tarnish resistance of EN can be improved by 15-60 minutes immersion in a warm (50 °C), 1% chromic acid solution -- US Patent 3,088,846 (1963).

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


A. It sounds like something is bleeding thru.
Question 1: Are they all the same alloy that is being plated?
Question 2: Are you positive that the EN thickness is the same on all parts? sounds like it is thin on the bad parts.

James Watts
- Navarre, Florida


A. Have you checked with your EN supplier? Sounds to me that someone is using too much over the tank side stabilizer or brightener. These products typically contain divalent sulfur compounds that are known to turn dark and stain EN deposits. Again, see your EN supplier!

milt stevenson jr.
Milt Stevenson, Jr.
Syracuse, New York


A. I believe Milt is correct, Stabilizer is probably the cause.
I'll go out on a limb and predict it is of the home brew nature.

Rob Coffie
Warren, Michigan, USA

January 3, 2008

A. Too bad I didn't see this post sooner. If that happens again, check your EN solution for NO3- using a test strip. I had that same problem up in Boston and traced it back to nitrates that were being dragged in from the bright-dip bath. We had to add another rinse step before EN submersion and it worked out fine.

Randy Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA



Tarnish of electroless nickel over brass or alum. when ultrasonic aqueous cleaning

May 7, 2009

We have thousands of parts Electroless Nickel plated and often they look fine before cleaning in a multi-stage ultrasonic cleaner. However, after processing in the ultrasonic cleaner (two stages of ultrasonics, one rinse, one dry --cleaner is Blue Wave Detergent #25-I, 4 parts purified (Seattle city water through filters) deionized water to 1 part liquid detergent). Many of the parts come out with brownish to blackish spots of irregular shapes. They kind of look like the variation you would get with water spots. Strangely, if we don't turn on the ultrasonics, we don't have this staining issue (though we have to soak them longer to get fingerprints, etc. off of them). We are trying to get them clean for oxygen service. The brownish/blackish spots can be buffed off but that is labor intense and thins the surface.

Ed Huncovsky
- Kirkland, Washington


Black patches on Nickel plated aluminium parts

May 21, 2014

Q. Hi,
We are facing black patches in High Phosphorous nickel plating on Aluminium parts.

We do plating twice, first time 25 to 30 micron electroless nickel, then Copper band soldering, then electrolytic nickel plating 3 to 5 microns.

What would be the root cause for this. Please help.

Santosh Musandi
Quality analyst - Bangalore, Karnataka, India

January 18, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. A medium phosphorous electroless nickel plated 17-4 PH stainless steel part on one of our handheld devices becomes tarnished very quickly. It is mainly a aesthetic problem but we would like to prevent this from happening.
Are there alternative finishes that look similar to electroless nickel that do not develop this problem or are there other options out there to prevent the electroless nickel finish to tarnish?

Dominik Scheffler
- Phoenix, Arizona, USA
  ^- Privately contact this inquirer -^

January 2017

A. Hi Dominik. Nickel-chrome plating would not tarnish, but it's hard to suggest what finishes would be appropriate without knowing anything else about the application. There may be a very good reason that electroless nickel is used instead of nickel-chrome plating.

It's fairly common to apply chromate-based tarnish preventers (Tarniban, for example) to electroless nickel plated components, but this is becoming less environmentally accepted; and without knowing what this handheld device is, it's hard to say whether it would be problematic from a human exposure perspective.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

February 6, 2017

A. Hi Dominik,
I've used anti-tarnish post-treatment dips on silver but I don't have experience using the dip on EN therefore I would recommend a 2-part PU clear coat.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California

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